Posted by: Adam Kay | January 8, 2013

Everyone hates rock snot, but those invasive purple flowers are pretty! By Liz Chambers and Adam Kay

During our field sampling in New Zealand this January, we’ve found that it’s easy to get excited about all of the beautiful landscapes and striking plants and animals. However, the more we learn about New Zealand ecology, the more we realize that we have to have a nuanced emotional response to what we see. New Zealand has many invasive species, many of which do substantial damage to native ecosystems. It turns out that much of the biology that catches our attention only exists in New Zealand because of recent introductions resulting from human activities.

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One funny experience happened on our first day sampling. We were walking from sampling Lake Rotoroa near the north end of New Zealand’s South Island. We stopped to admire some beautiful pink flowers and Liz said “Oh, cool” and snapped a picture. Maurine Neiman, our collaborator from the University of Iowa who has worked for over a decade on New Zealand snails and the evolution of sexual reproduction, replied with a tender smile, “Oh, those are actually invasive, haha”. Liz went silent and Adam chuckled. It turns out that the plant with the pretty pink flowers, Digitalis purpurea (=foxglove), was introduced to New Zealand by European settlers some time before 1860.

Like other islands, New Zealand is vulnerable to invasive species. Islands tend to have lower biological diversity than continental areas, in part because they’re distant from populations of potential colonizing species. New Zealand has been isolated from other land masses for about 80 million years. On the one hand, this isolation has produced distinctive and interesting species. For example, there is a large cricket-like creature, the weta, that fills the ecological niche of mice (New Zealand is known as “a land without teeth”, because there are no native land mammals – Adam and I thought this matched the countries overall cute persona!). On the other hand, the isolation likely has also resulted in the evolution of species that are susceptible to competition or predation from continental species that have traits honed by a long history of ecological challenge in a biodiverse environment.

One well-known example of vulnerability involves the flightless birds of New Zealand. Several species of New Zealand birds (including the weka (short video) and of course the kiwi), likely evolved flightlessness because they didn’t face predation pressure from land mammals. However, the introduction of many land mammals since European colonization has resulted in widespread population declines for these flightless birds. The kiwi, the iconic and lovable symbol of New Zealand, is particularly vulnerable to predation by possums and stoats (a member of the weasel family), which are possibly the most despised pests among the people of New Zealand. (According to the New Zealand Department of Conservation, 9 out of 10 north island kiwi chicks born in the wild will die before they are a year old. However, conservation efforts in kiwi sanctuaries all over the country have increased survival rates of kiwi to nearly 70%!)

Liz with her possum purchases

Liz with her possum purchases

We’ve already experienced the frustration of New Zealanders with the scourge of invasive species. One example was when we were in Hokitika on the west coast. We stopped at ‘The Possum People’, an interesting store that specializes on vests, hats, shoes or other odds and ends made with pelts from possums that were hunted or trapped as part of eradication efforts.  While we were perusing the selection of giant awesome possum vests some native New Zealanders came in and one of the women asked how many ‘pests’ it took to make a blanket hanging on the wall behind the register. The cashier replied, “About 40”, and a chorus of four women responded “Good!” and the room erupted with lighthearted laughter. The people here are happy to see them go, and it’s amazing how they are able to make a new market out of the pelts.

A stunning landscape near Arthur's Pass filled with beautiful but invasive purple lupines

A stunning landscape near Arthur’s Pass filled with beautiful but invasive purple lupines

Because we’ve learned that there are so many invasive species are so common in New Zealand (e.g., 42% of bird species are non-native!), we now end up analyzing vistas as we take pictures and videos. While driving through Arthur’s Pass (a 2000m pass in the center of the south island) we entered a beautiful valley surrounded by snow-caped mountains and filled with vibrant blue tide pools sprinkled with a sea of purple plants. Adam thought the plants looked like the purple lupines in Minnesota grasslands. We left hoping that this beautiful landscape was a result of convergent evolution in nitrogen limited soil (because purple lupines are nitrogen fixers) and that New Zealand was part of its native distribution. It turns out that the purple flower is Lupinus polyphyllus, a plant first documented as naturalized in New Zealand in 1958. According to Wikipedia, it may have been introduced by tour bus drivers interested in adding color to the landscape. Today it threatens native plants near river beds in the South Island.

One obvious landscape transformation related to invasive species has been the conversion of natural rainforest into pasture lands for sheep and cows. In 1773 and 1777, sheep were brought to New Zealand by James Cook (the first person to circumnavigate New Zealand). Cook gave sheep as a gift to the Maori, the native people of New Zealand. However, it wasn’t until the establishment of substantial European settlements (~1840) that sheep farming spread throughout the island. Since that time, sheep have been an integral part of the New Zealand economy. Although sheep numbers have declined over the last two decades there are still many more sheep (40 million) than people (4 million) in New Zealand (here is a nice video of a common sight). Sheep are everywhere we’ve been, and they are not to be messed with (as Liz learns here).

New Zealand will continue to be plagued by invasive species, but further spread can be curtailed using public awareness campaigns. One invasive species relevant to our work is Didymosphenia geminata or Didymo, which has been given the charming little nickname of “rock snot”. It is a diatom that covers rocks on the bottom of rivers and lakes in long stringy mats. It changes water quality and alters lake habitats, which in turn harms native invertebrates and fish species. Didymo has spread over the South Island since the first reported sighting in 2004. It can be spread by just one drop of water, so we clean all our equipment in 5% detergent between sites. Luckily we haven’t seen rock snot in any of the lakes we’ve sampled, although we’ve read that it’s been found in many rivers and lakes in the South Island.

Ironically, our study species Potamopyrgus antipodarum is one of the few native New Zealand species that is actually invasive in other parts of the world. A recent article in The New Zealand Herald declared that this tiny mud snail is taking over the world! It’s now found in Europe Australia, America and Asia. Although it’s not entirely clear why it is a successful invader, it may be due to the fact that it can live outside its natural aquatic habitat for long periods of time, it is very small (about the size of a pencil lead!), and some members can reproduce asexually (which means that even a single snail can make a lot of offspring quickly). Maurine has had a few conversations with New Zealanders who have expressed surprise and maybe even some pride when they hear that a native species is invasive somewhere else. It must be emotionally draining to live in such an ecologically fragile country.



  1. Wow, great post! This is a fascinating question, I think–can one, in good conscience, appreciate the beauty of non-native species in a landscape? (my opinions on the topic of invasive species are heresy for most ecologists). I think we need to develop an Aquinas Scholars course with an ethicist…

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